At physiological concentrations, 17beta estradiol enhances endothelium-dependent vasodilation when administered to postmenopausal women, but by an unknown mechanism. In order to assess the contribution of nitric oxide to estradiol's vascular effects, we measured coronary epicardial and microvascular responses to the peak vasodilating dose of acetylcholine (3-300 mcg/min x min) before and after intracoronary infusions of estradiol (75 ng/min x 15 min), with a repeat of this testing following inhibition of nitric oxide synthesis with intracoronary N/G-monomethyl-L-arginine (L-NMMA:64 mcM/min x 5 minutes) in 17 estrogen-deficient postmenopausal women. Ten women showed enhancement of acetylcholine-stimulated microvascular dilation and prevention of acetylcholine-stimulated epicardial coronary artery vasoconstriction following infusion of estradiol. Following L-NMMA, these vascular responses were completely inhibited and virtually identical to the effects of L-NMMA in the remaining 7 women who did not show enhancement of endothelium-dependent vascular effects by estradiol.Of note, those 10 women who did show improvement in endothelium-dependent vascular responses following estradiol had the most abnormal baseline responses to acetylcholine, suggesting impaired nitric oxide bioactivity prior to estradiol administration. In the remaining 7 women who did not show enhancement of endothelium-dependent vascular effects with estradiol, the baseline acetylcholine-stimulated vascular responses prior to estradiol were significantly better, suggesting more preserved nitric oxide bioactivity. We conclude that estradiol enhances nitric oxide bioactivity in individuals with impaired nitric oxide bioactivity. Because nitric oxide has multiple biological effects that may inhibit atherogenesis, estrogen therapy may be of particular benefit to those individuals at greatest risk for atherogenesis because of impaired endothelial nitric oxide synthesis and release.